Plastics are used in many fields such as the automotive industry, mainly due to their great strength and low weight. Special demands are made on screw connections used to obtain a detachable connection of plastic components, demands that conventional sheet metal or wood screws either do not meet satisfactorily or not at all.
Thread-forming or thread-rolling screws are known in the state of the art and are subsumed under the term “wood screws”. They are comprised primarily of a head and a screw core (i.e. a shaft) in the form of an elongated cone or pin that tapers to a point at the tip of the screw. The screw core is surrounded by a helical peripheral cutting edge projecting radially from the thread root, which transitions into the two flanks of the cutting thread with a sharp bend. The flanks in turn are provided with a constant pitch from the thread root to the crest, resulting in a uniform thread angle between the two flank lines. Screws of this kind are also used to screw plastics together.
In addition, competition focusing on light-weight construction in the automotive industry has resulted in an ever-growing range of applications for plastics, including increasing use of physical or chemical foams which replace the plastic material with air (for e.g., nitrogen or carbon dioxide). In the case of physical foams produced using a MuCell® foam injection molding process or similar processes, substances such as nitrogen, carbon dioxide or a chemical blowing agent are injected under pressure in a supercritical state into the molten plastic and uniformly distributed. After injection into the unpressurized mold, the gas separates from the smelt and forms a fine-celled foam structure. The omission of holding pressure and the reduced viscosity, minimizing or eliminating sink marks and strain are the primary aspects in support of this trend.
When used in plastics, including foamed plastics, all known screws have the disadvantage of damaging the plastic they are driven into, for instance by rupturing the screw-in openings or by stress cracking. At the same time they also require relatively high screw-in torque. In screw connections, when MuCell® injection molded parts are used, the compact layer is destroyed, with the result that a conventional screw completely loses its hold in the remnants of the foam layer.